The present invention relates generally to open superconductive magnets, and more particularly to an open superconductive magnet having a cryocooler coldhead.
Magnets include resistive and superconductive magnets which are part of a magnetic resonance imaging (MRI) system used in various applications such as medical diagnostics. Known superconductive magnets include liquid-helium-cooled, cryocooler-cooled, and hybrid-cooled superconductive magnets. Typically, the superconductive coil assembly includes a superconductive main coil surrounded by a thermal shield surrounded by a vacuum enclosure. A cryocooler-cooled magnet typically also includes a cryocooler coldhead externally mounted to the vacuum enclosure, having its first stage in solid conduction thermal contact with the thermal shield, and having its second stage in solid conduction thermal contact with the superconductive main coil. A liquid-helium-cooled magnet typically also includes a liquid-helium vessel surrounding the superconductive main coil with the thermal shield surrounding the liquid-helium vessel. A hybrid-cooled magnet uses both liquid helium (or other liquid or gaseous cryogen) and a cryocooler coldhead, and includes designs wherein the first stage of the cryocooler coldhead is in solid conduction thermal contact with the thermal shield and wherein the second stage of the cryocooler coldhead penetrates the liquid-helium vessel to recondense "boiled-off" helium.
Known resistive and superconductive magnet designs include closed magnets and open magnets. Closed magnets typically have a single, tubular-shaped resistive or superconductive coil assembly having a bore. The coil assembly includes several radially-aligned and longitudinally spaced-apart resistive or superconductive main coils each carrying a large, identical electric current in the same direction. The main coils are thus designed to create a constant magnetic field of high uniformity within a typically spherical imaging volume centered within the magnet's bore where the object to be imaged is placed.
Open magnets, including "C" shape and support-post magnets, typically employ two spaced-apart coil assemblies with the space between the assemblies containing the imaging volume and allowing for access by medical personnel for surgery or other medical procedures during magnetic resonance imaging. The open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design. The sharpness of an MRI image depends, in part, on the magnetic field in the imaging volume being time-constant and highly uniform, such magnetic field suffering time and spatial deformation caused by vibrations imparted to the coil assemblies by the presence of a cryocooler coldhead. What is needed is a design for an open superconductive magnet having a cryocooler coldhead which reduces vibrations imparted to the coil assemblies by the cryocooler coldhead and hence which improves the sharpness of an MRI image.